It is known that the performance of gas turbines, in terms of efficiency and obtainable power, are intrinsically bound to the maximum temperature of the thermodynamic cycle, that is to the temperature of the hot gases in contact with the metallic walls of its elements, in particular turbine vanes of the first rotor and stator stage.
The super-alloys used in the construction of elements exposed to high temperature are therefore stressed to their technological limits and are consequently subject to processes of oxidation, corrosion and erosion made continuously more taxing by the increasingly high running temperature and use of lower quality fuels.
The need to coat the surface of such elements with elements capable of preserving their structure and prolonging their reliability has therefore arisen.
It is known the coating of super-alloy elements with metallic compositions, for example of the McrAlY type, where M may be Nickel, Cobalt or Iron.
These latter are generally applied by plasma spraying both in air (APS) and in vacuum or at low pressure (VPS or LPPS) or thermal sprayed by oxygen-fuel system (HVOF).
The MCrAlY type compositions are normally used to protect the substrate from oxidation and corrosion.
In particularly taxing environments, such as for example in the case of first stage turbine vanes, the McrAlY composition is generally associated to a overlaid ceramic thermal barrier.
The MCrAlY compositions have the task of protecting the super-alloy substrate from oxidation, but also of anchoring the thermal barrier to it.
Indeed, the aluminium present in the McrAlY composition, coming into contact with the oxygen, oxidises selectively forming a layer of α-Al2O3.
Such oxide, being very compact and chemically stable at the running temperatures of the turbines, between 900° C. and 1100° C., prevents the further diffusion of oxygen towards the underlying metallic substrate protecting the super-alloy element from oxidation.
Furthermore, the anchoring function between substrate and thermal barrier is performed both mechanically, by protrusions, commonly called pegs, generated by the oxidation of Y, Re and Hf, if present, and by diffusion of Al3+ ions in the thermal barrier itself.
The MCrAlY type composition can be assimilated macroscopically to a metallic alloy constituted mainly by a lattice γ, comprising prevalently Ni, Co and Cr, in which are dispersed particles of a second Aluminium rich phase β, in particular in the form Ni—Al and/or Co—Al.
In oxidising environment, the aluminium of phase β reacts with the oxygen originating the protective flake of α-Al2O3.
Generally a NiCoCrAlY composition presents better features with respect to a NiCrAlY in terms of coating stability, ductility and resistance to corrosion.
The microstructural features of a coating composition and therefore its performance above all in terms of durability are strongly influenced by the elements which constitute it and by their content by weight.
The constituting elements can be classified in two main categories: reactive elements and noble elements.
The first, mainly Y, Si and Hf, form oxides in the boundary zone with alumina, by reaction with oxygen in the environment. Such oxides are responsible for the formation of preferential routes for oxygen which reacts in turn with the Al of the coating to form an alumina flake capable of incorporating the previously formed oxides stabilising the protective flake. In the presence of overlaying thermal barrier, these act mechanically as anchoring between the alumina and the thermal barrier itself.
Another main function of the reactive elements is to slow down the diffusion of the aluminium and of the chromium of the coating outwards preventing depletion and therefore prolonging life.
The presence of reactive elements also helps to prevent the segregation of sulphur at the interface between the alumina flake and the coating. The presence of chromium is effective against the hot corrosion which, with the formation of embrittling sulphides raises the ductile-brittle transition temperature (DBTT).
The noble elements, such as Re and Pt, in virtue of their large dimensions and higher density can interact as diffusive barriers for carrying aluminium and chromium outwards but also oxygen inwards. In that way, the growth of the alumina flake is thus slowed down as the depletion of the phase β, which otherwise would cause exhaustion of the aluminium reserve and loss of protective efficiency of the coating with consequent formation of microcavities and therefore thermo-mechanical fatigue phenomena in the super-alloy element.
Currently there are known different types of MCrAlY type compositions for coating super-alloy articles.
From U.S. Pat. No. 5,268,238 it is known, for example, a composition of the MCrAlY type with possible additions of Re, Si and elements such as Hf, W, Ta, Ti, Nb, Mn and Zr.
Furthermore, from U.S. Pat. No. 6,756,131, it is known the use of a composition of the MCrAlY type resistant to high temperatures comprising nickel, cobalt, chromium, aluminium, yttrium, and rhenium.
In particular, as shown in W. Beele et al. (Surface and coating Technology, 1997, 94-95), Rhenium is capable of slowing down the depletion of the phase β, by forming a chromium rich phase σ immediately under the Al2O3 flake, and a phase α, even richer in chromium than phase σ. Such phases compensate the depleted zones and thus prevent the embrittlement of the coating, hindering the formation of voids. Table 1 shows the chemical composition of the phases present in a generic NiCoCrAlYRe composition.
TABLE 1PhaseNiCoCrAlYReγ3924288—1β5011732 ——α5481——9σ111950298
The metallic rhenium being a large and heavy atom (atomic weight=186.2) behaves as a noble element interfering on diffusivity, inhibiting the growth of the flake and therefore delaying the depletion of the phase β.
Disadvantageously, the rhenium, when present in contents higher than 3%, however manifests an embrittling effect of the coating; such embrittling effect of Rhenium therefore reduces, in practice, applicability.
Furthermore, the compositions of the MCrAlY type comprising rhenium, when applied onto cobalt based substrates, show an even more marked embrittlement also with minimum contents of rhenium, and therefore cannot be successfully used on all cobalt based super-alloy components.
The development of new coating compositions free from the drawbacks of the prior art is therefore a fundamental need in the super-alloy element protective coating technology sector.
U.S. Pat. No. 6,183,888 describes a process for the manufacture of a protective coating of super-alloy articles which envisages the deposit of an alloy powder comprising at least Cr, Al and an active element with a residual open porosity followed by the deposit of a further layer comprising at least one metal of the platinum group, such as for example ruthenium, rhodium or iridium, so as to fill the residual open porosity. The process described in U.S. Pat. No. 6,183,888 shows a deposition phase of a layer of iridium on a layer of MCrAlY alloy then followed by a diffusion phase by means of thermal treatment. Such process is however complex, long and costly.